In heat-treating of solid ferrous alloys within a furnace, gas nitriding is one of the often-used processes to achieve high surface hardness, to improve fatigue life, and increase antigalling properties. Nitriding is performed by placing metal parts in the furnace, heating the metal to a proper temperature between 495° C. and 565° C., and holding the metal in contact with a nitrogenous gas, typically anhydrous ammonia. Ammonia dissociation in the furnace atmosphere is critical for the quality of the heat treated parts, as discussed in Floe, U.S. Pat. No. 2,437,249, “Method of Nitriding”, and Rose, et al., U.S. Pat. No. 4,264,380, “Nitride Casehardening Process and the Product Thereof.” Nitriding capability of the nitriding atmosphere may also be expressed as nitriding potential, as suggested in “Aerospace Material Specification Standard AMS 2759/10”. Nitriding Potential measurement requires the measurement of partial pressure of ammonia and the partial pressure of hydrogen as a result of dissociated ammonia in the furnace atmosphere exhaust. Gas nitrocarburizing processes such as described in Heminghous, U.S. Pat. No. 4,776,901, “Nitrocarburizing and nitriding process for hardening ferrous surfaces”, also require ammonia gas dissociation in the furnace measurement.
Because ammonia dissociation information is so important, automatic means of measuring ammonia dissociation in a heat-treat atmospheres were developed and are being used. Those methods usually consist of photometry of ammonia in the infrared range while ammonia is still in gaseous form, or measurement of ammonia combustion on a catalyst thermal reaction.
The principal of infrared photometry is based on measurement of the intensity of infrared light transmitted, absorbed, or reflected from a gas sample, and compared with a reference light intensity. Such a process has many disadvantages, such as interference of other gasses in the mixture, drift of the instrument, and need of frequent calibration, thereby incorporating expensive certified gasses. One of the biggest obstacles of using infrared ammonia concentration measurement is the necessity to have extremely rigorous maintenance procedures in place to keep the optics and sensors clean.
Another automatic ammonia dissociation concentration measurement of a heat-treat atmosphere method involves determining the individual gas concentration in a mixture of gasses by a thermal reaction heat measurement, where the heat is generated by ammonia being burned on a catalyst. This approach is also difficult to perform reliably in a factory setting.
Another automatic ammonia dissociation measurement of a heat-treat atmosphere method involves determining the concentration of gasses that are a product of dissociated ammonia, such as hydrogen, and then calculating actual dissociation. This approach, however, also uses sensors that do not always endure the harsh factory environment.
Measurement of dissociated ammonia in the gas mixture is often done by sampling a predetermined amount of gas, then dissolving ammonia in the gas form into a liquid form, and determining the concentration of ammonia in solution. Such methods are typically manual operations, but Keil, et al., U.S. Pat. No. 5,801,296 “Process for automated measurement of ammonia content in a gas mixture,” and Keil, et al., U.S. Pat. No. 5,767,383, “Apparatus for automated measurement of ammonia concentration in a gas mixture,” disclose a method of automating ammonia contents measurement in the gas sample by dissolving ammonia into water. To reveal the amount of dissolved ammonia, the patented apparatus and process use a tubular-shaped measurement vessel and a technique to measure pressure created by a water column, or measuring the height of water column. However, the pressure transducer port is prone to water contamination resulting in an error in the sensed differential pressure and, therefore, an error in determining the ammonia concentration in a gas mixture. A number of measures to prevent water entrapment were taken. The methods described in the above patents include the steps of providing a fifth solenoid valve for draining the entrained water, and providing inlet ports located at exactly pre-set angles.